“We’ve lost about 80 percent of the living coral in the Caribbean and 50 percent in the western Pacific,” says coral biologist Nancy Knowlton of the Smithsonian Institution in Washington, D.C. “So a better understanding of what keeps corals healthy is essential.”

In reefs, corals and the seaweed algae that form lawns or shrubby thickets compete for light and space. As coral reefs decline from pollution, overfishing, climate change and other insults, biologists have seen swaths of seaweed take over. Lush seaweed intrusions repel or smother larval corals, accelerating what Hay calls the reef “death spiral.”